Specific crystallographic site occupancy induced water stability: towards facilitating ‘aqueous processing’ of ‘layered’ Na-transition metal oxide cathodes for Na-ion batteries

Abstract

Severe compositional/structural instability of ‘layered’ Na-transition metal (T_M) oxide cathode materials for Na-ion batteries upon exposure to air/water renders their handling/storage challenging and mandates the use of toxic/hazardous-cum-expensive chemicals, like N-methyl pyrrolidone, as the solvent for electrode preparation; viz., ‘non-aqueous processing’. Against this backdrop, since the major mechanism associated with ‘air/water-instability’ involves spontaneous intercalation of water-based species (especially, H⁺) and simultaneous Na-extraction from the lattice of ‘layered’ Na-T_M-oxide via tetrahedral sites of the Na layer, the present study reveals that precise positioning of a suitable cation at some of the tetrahedral sites can hinder the same, in truly significant terms. As also demonstrated here, the vastly improved air/water-stability can even facilitate health/environment-friendly ‘aqueous processing’ of electrodes (viz., using water as the solvent), with absolutely no compromise on the electrochemical behaviour/performance. A combination of experimental results/observations/inferences, bond valence sum analysis and density functional theory simulation has established that a small fraction of d⁰ Ti⁴⁺, having non-existent crystal-field stabilization energy, is present in the tetrahedral sites of the Na-layer of Na(Li_0.05Ni_0.3Ti_0.5Cu_0.1Mg_0.05)O₂; which significantly hinders the insertion of water-based species into the Na-T_M-oxide lattice (and concomitant Na-extraction) upon air/water-exposure by directly impeding the transport pathway. This, in turn, bestows Na(Li_0.05Ni_0.3Ti_0.5Cu_0.1Mg_0.05)O₂ with exceptional air/water-stability. More importantly, the excellent ‘water-stability’ enables ‘aqueous processing’ of the electrodes, which still exhibit excellent electrochemical behaviour/performance in Na ‘half’, as well as Na-ion ‘full’, cells. In the broader context, such an elimination of the requirements for toxic/hazardous-cum-expensive ‘non-aqueous’ solvents/binders for electrode preparation and the associated learning from a materials-chemistry perspective are important steps towards the development of sustainable and high-performance Na-ion batteries.